Halocyclopentadienes, and particularly hexachlorocyclopentadienes, are useful compounds for the preparation of DielsAlder adducts. For example, the adduct with maleic anhydride, commonly known as chlorendic anhydride is useful in the preparation of polyester resins. The adduct with furan gives products which are useful as fire-retardant additives to many otherwise flammable resins.
Commercial grades of hexachlorocyclopentadiene contain impurities such as hexachlorobutadiene and octachlorocyclopentene, neither one of which will adduct with dienophiles such as maleic anhydride. These chlorocarbon impurities, as well as impurities arising from incomplete reaction give rise to objectionable odors and vapors which may attack and irritate the eyes and sensitive membranes of the respiratory tract of workers handling this material. The chlorocarbon impurities also act as skin irritants giving rise to a type of dermatitis. Moreover the chlorocarbon impurities can also form objectionable and resinous discolorations and decomposition products on subsequent reaction of the adducts with polyols and the like. The chlorocarbon impurities further, may cause corrosion damage to the metal processing equipment if allowed to remain in the hexachlorocyclopentadiene or adducts prepared therefrom.
The prior art accomplished only partial removal of these contaminants from the commercial product by various methods including one-stage fractional distillation, and by extensive recrystallization of the adducts. Moreover, these methods are time consuming and expensive and not economically adapted to a commercial operation.
In U.S. Pat. No. 3,214,444 it has been proposed to purify chlorendic anhydride and related compounds by azeotropic distillation of the halocarbon impurities wherein a material which will form a constant minimum boiling mixture with the halocarbon impurities is added to the crude anhydride and subsequently distilling off said constant minimum boiling mixture at a temperature low enough to prevent unwanted by-products from forming in the anhydride product. This method while suitable for chlorendic anhydride, is not adaptable to the purification of hexachlorocyclopentadiene and related compounds.